Add persistent storage backend (SQLite/PostgreSQL)

[mkmkkkkk]

Problem

Currently SpendTracker only records transactions in memory. All spending history and policy state is lost on restart. This is fine for development but unacceptable for production deployments.

Proposed Solution

Implement a StorageBackend interface with the adapter pattern:

interface StorageBackend {
  recordTransaction(tx: Transaction): Promise<void>;
  getTransactions(filter: TransactionFilter): Promise<Transaction[]>;
  getSpendingTotal(agentId: string, window: TimeWindow): Promise<number>;
}

Implementation Plan

1. Phase 1: SQLite — Zero-config, embedded, perfect for single-node deployments
2. Phase 2: PostgreSQL — For multi-instance / production environments
3. Phase 3: Migration tooling — Schema versioning with automatic migrations

Acceptance Criteria

• StorageBackend interface defined
• SQLite adapter implemented and tested
• Existing in-memory tracker refactored to implement the same interface
• Configuration via environment variables (PAYSENTRY_STORAGE=sqlite|memory)
• Documentation updated with storage configuration guide